Haemophilus somnus: investigations of its potential role in bovine mastitis.

Mammary glands of 6 lactating Holstein cows were inoculated with Haemophilus somnus strain 43826. Three cows developed chronic mastitis and shed bacteria for up to 1 year. Three cows developed acute gangrenous mastitis, with evidence of bacteremia and endotoxemia. Cows with gangrenous mastitis had lower somatic cell counts early after inoculation in affected quarter secretions compared with those in cows that developed chronic mastitis. Cows with gangrenous mastitis developed hypocalcemia, hypoalbuminemia, azotemia, hyperbilirubinemia, mildly increased serum aspartate aminotransferase and creatine kinase activity, and a marked neutropenia with a degenerative left shift. Histopathologic examination of gangrenous quarters revealed edema, necrosis, and vascular thrombosis with few inflammatory cells. A limited survey failed to recover H somnus from dairy cows with clinical mastitis or from mammary secretions from 41 beef cattle at an abattoir.